1. Field of the Invention
The present invention relates to a method for manufacturing a graded index type plastic optical-fiber and a manufacturing apparatus therefor.
2. Description of Background Information
For manufacturing this type of plastic optical fiber, several methods have been disclosed, for example, in published Japanese Patent Application Nos. HEI 5-507610 and HEI 7-27928.
In the method described in Application No. HEI 5-507610, a cladding is formed of a cylindrical tube having a hollow central part which is filled with a liquid mixture consisting of a monomer and a compound having a high refractive index. The liquid mixture is then heated or irradiated with energy-carrying rays, thereby generating and propagating a polymerization reaction, preferentially on a predetermined site. The compound having a high refractive index thus forms a density gradient in the resulting product which is used as a base material. The base material is melted by heating and is drawn so as to continuously vary its refractive index along the radial direction. This base material constitutes the core part of the plastic optical fiber.
In the method disclosed in Application No. HEI 7-27928, a polymerized fiber material is melted by heating and then spun to form a fiber. The fiber thus obtained is soaked in a solution containing a non-polymerizable compound having a refractive index lower than that of the initial polymer, whereby this non-polymerizable compound is impregnated and diffused into the fiber. In this manner, a plastic optical fiber having a graded index is formed.
However, in the method disclosed in Application No. HEI 5-507610, the fiber is formed by heating, fusing and drawing the base material. The fiber length producible from one base material is thus limited, i.e., the fiber length depends on the size of the base material.
In order to manufacture a longer fiber, the base material must have a thicker diameter or must be of a longer dimension size. When the base material is thickened, the subsequent drawing process becomes difficult because of insufficient heating and fusing. When the base material is lengthened, the material should have a uniform graded index and constant transmission loss in the longitudinal direction. However, the manufacture of such a material requires stringent conditions to be satisfied for controlling the reactions and polymerization. When manufacturing a plastic optical fiber having a length over 1,000 m and a diameter of 1 mm, a one-meter base material should have a diameter over 30 m.
Further, this method is a batch system in which a base material is first prepared, drawn, and then transformed into an optical fiber. The running ratio of the process therefore is not good and, as the production increases, the manufacturing facilities must be scaled up greatly.
In the method disclosed in Application No. HEI 7-27928, the fiber is continuously manufactured, so that it can be made with a desired length. However, after the fiber is manufactured, it has to enable a non-polymerizable compound to diffuse from its surface inwards. A large molecular-size, low refractive-index compound is, however, difficult to diffuse into such a fiber, so that the refractive index is poorly graded.
In order to diffuse a large molecular-size, low refractive-index compound into the fiber, the compound and the fiber must be heated to a high temperature. However, intense heating tends to melt and cut the fiber and the fiber manufacturing becomes inconsistent.
In view of the above, an object of the invention is to provide a new method of manufacturing a graded-index type, plastic optical fiber and a manufacturing apparatus therefor. This method attains a good running ratio of the process and minimizes the scaling-up of installation required for production increase. Further, the plastic optical fiber is easily provided with a desired graded index. Moreover, the graded-index fiber can be manufactured, continuously and constantly, in a desired length with a constant quality.